That only seems to work on btrfs, XFS, (and maybe now or very soon) ZFS and bcachefs: "[duperemove] simply finds candidates for dedupe and submits them to the Linux kernel FIDEDUPERANGE ioctl." [1] (aka BTRFS_IOC_FILE_EXTENT_SAME), and this ioctl "performs the 'compare and share if identical'" (and locking etc.) work [2]. But on those filesystems, that is a nice feature, plus it lets the tool get away with a weak hash like murmur3.

1: http://markfasheh.github.io/duperemove/duperemove.html 2: https://manpages.debian.org/bookworm/manpages-dev/ioctl_fide...